# How to fix the upper asymptote in sigmoid?

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Srilatha Raghavan on 31 Jul 2020
Hello, I have a sigmoid equation. My qestion is in a comment here:
The problem I am facing is, this sigmoid equation is a power curve and needs to reach a certain fixed height at the rated speed (variable 'a'). But I am not able to adjust my upper assymptode anything above 650, even by forcing my a to be an independent variable and also by replacing a by 900. Since, I am new to sigmoid, I would like some help to rewrite this piece of code to make my sigmoid reach 'a'. Also, I def need my xon, xoff and the everything about the curve from below to remain the same.
TIA
xon =3;
xrated = 12.5;
off=25;
a=900;
ft = fittype(@(b,c,x) mysigmoid(b,c,x,xon,xrated,a));
mdl = fit(x',y',ft,'start',[5 20]);
function yhat= mysigmoid(b,c,xon,xrated,a)
x= max(xon,min(x,xrated))
yhat = a./(1+exp(-(x-c)/b))- a./(1+exp(-(xon-c)/b));
end

Shae Morgan on 31 Jul 2020
Your call to mysigmoid has 6 input arguments (b,c,x,xon,xrated,a; the last of which is your "a"), but your defined function only accepts 5 (b,c,xon,xrated,a). you're missing the 'x' input argument in your function definition.
This is likely leading to your issues with manually setting the "height" of the sigmoid function.

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Shae Morgan on 31 Jul 2020
your attached code doesn't run without the data "windspeed' and "tgeneration" could you provide some sample data for that?
Shae Morgan on 3 Aug 2020
your code still doesn't run after reading in these X and Y values. Please provide a code that will run so it can be used.
ex. missing 'b', 'c', and 'x' values.
Srilatha Raghavan on 4 Aug 2020
I figured this out! If the data that is used to initialize the fit does not reach the required upper symptote, we cannot expect the model to do what want it to with our data. It was a basic understanding of the sigmoid and curve fitting toolbox. Anyways thank you.